Surface topography of acetylcholinesterase in Langmuir and Langmuir-Blodgett films

The surface topography of the enzyme acetylcholinesterase was studied at the air/aqueous and the air/solid interfaces using the Brewster angle and the atomic force microscopies, respectively. Surface potentials of the enzyme monolayer have been measured in conjunction with the surface pressure. The surface potential and the surface dipole moment data show that the orientation of the molecular dipoles occurs before the orientation of the hydrophobic groups of the acetylcholinesterase monolayer. The variations of the surface potential observed at large molecular area suggest the presence of domains in the film. The Brewster angle images confirm the formation of domains at the air/aqueous interface. The size of these domains increases with decreasing the molecular area. Furthermore, the Brewster angle microscopy allowed us to detect a reversible formation of the domains upon the compression and the decompression of the monolayer. On the other hand, the atomic force microscope images of the Langmuir-Blodgett films show that the enzyme molecules are more close-packed at a surface pressure of 25 mN/m than at 20 mN/m. Size measurements of the enzyme particles indicate that acetylcholinesterase has an ellipsoidal shape and that the tetramer form of this enzyme is the most abundant.